Double-tapping jump button to fly cannot be disabled/reconfigured

Reported due to a comment on MC-203401.

The bug

Flying via double-tapping the "jump" button cannot be bound separately from the "jump" button itself. This means there is no way (outside of changing gamemode) to not fly when tapping the "jump" button in quick succession, which may be required in certain circumstances such as dealing critical hits in tight spaces.

This may be considered a feature request but for a control to be absent from configuration does not seem right.

How to reproduce

1. In Creative mode, double-tap your usual jump button while grounded

2. Note that this causes flying

3. Change the jump keybind to an unused key such as K

4. Double-tap this new key and note that you still start flying, demonstrating that it follows the "jump" key

Expected behaviour

The double-tap-to-fly behaviour would not follow the jump button.

Actual behaviour

This functionality is hardcoded to always be stuck with the jump button. There exists no option to change it, nor is it a different rebindable control that happens to share the jump key's place by default.

How to fix

A preferable solution would be for there to be a dedicated keybind for flying, which would default to the spacebar key just like jumping does. When both are bound to spacebar, pressing spacebar once would cause the player to jump as usual whereas pressing it twice would activate flying. If the fly key and jump key do not share a keybind, the dedicated flying key would immediately activate or deactivate flying upon a single press.